Support source-phase imports in the acorn/terser optimizer pipeline

When -sSOURCE_PHASE_IMPORTS=1, emcc emits
```js
import source wasmModule from './foo.wasm';
```
in its JS runtime. At -O2/-O3/-Os/-Oz the emitted JS is run through
tools/acorn-optimizer.mjs which currently fails with a SyntaxError at
parse time because acorn 8.x does not yet understand the source-phase
imports proposal (https://github.com/tc39/proposal-source-phase-imports).

Wire in the acorn-import-phases plugin so acorn can parse the syntax,
and pull in the matching terser support (downstream of emscripten-core/terser#1)
so the round-trip through from_mozilla_ast / to_mozilla_ast preserves
the `phase` keyword on the way back out. Without the terser side,
terser would silently drop the keyword and the host would return the
module's exports namespace instead of a WebAssembly.Module, changing
runtime semantics.

* package.json: add acorn-import-phases dependency.
* tools/acorn-optimizer.mjs: extend acorn with the plugin and use the
  extended parser at the parse site.
* third_party/terser/terser.js: rebuilt from the emscripten-core/terser
  branch with source-phase imports support (PR
  emscripten-core/terser#1, the v5.18.2
  downstream port of upstream terser PR
  terser/terser#1682).
* test/js_optimizer/sourcePhaseImports{,-output}.js: new fixture that
  feeds two `import source` declarations through the JSDCE pass and
  checks the keyword survives.
* test/test_other.py: register the fixture in test_js_optimizer, and
  parametrize test_esm_source_phase_imports across no-args and -O2 to
  exercise the optimizer pipeline.

Author: guybedford

package-lock.json

@@ -18,6 +18,7 @@
   },
   "dependencies": {
     "acorn": "^8.15.0",
+    "acorn-import-phases": "^1.0.4",
     "google-closure-compiler": "20260429.0.0",
     "html-minifier-terser": "7.2.0"
   },

package.json

@@ -0,0 +1,9 @@
+import source wasmModule from "./foo.wasm";
+
+import source otherModule from "./bar.wasm";
+
+function use() {
+  return [ wasmModule, otherModule ];
+}
+
+use();

test/js_optimizer/sourcePhaseImports-output.js

@@ -0,0 +1,11 @@
+// Source-phase imports (https://github.com/tc39/proposal-source-phase-imports).
+// The acorn optimizer must parse these via the acorn-import-phases plugin and
+// preserve the `source` phase keyword through the terser from_mozilla_ast ->
+// print round-trip used at -O2+.
+import source wasmModule from './foo.wasm';
+import source otherModule from './bar.wasm';
+
+function use() {
+  return [wasmModule, otherModule];
+}
+use();

test/js_optimizer/sourcePhaseImports.js

@@ -421,11 +421,15 @@ def test_esm(self, args):
     self.assertContained('Hello, world!', self.run_js('hello_world.mjs'))
 
   @requires_node_25
-  def test_esm_source_phase_imports(self):
+  @parameterized({
+    '': ([],),
+    'O3': (['-O3'],),
+  })
+  def test_esm_source_phase_imports(self, args):
     self.node_args += ['--experimental-wasm-modules', '--no-warnings']
     self.run_process([EMCC, '-o', 'hello_world.mjs', '-sSOURCE_PHASE_IMPORTS',
                       '--extern-post-js', test_file('modularize_post_js.js'),
-                      test_file('hello_world.c')])
+                      test_file('hello_world.c')] + args)
     self.assertContained('import source wasmModule from', read_file('hello_world.mjs'))
     self.assertContained('Hello, world!', self.run_js('hello_world.mjs'))
 
@@ -3012,6 +3016,7 @@ def test_extern_prepost(self):
     'minifyGlobals': (['minifyGlobals'],),
     'minifyLocals': (['minifyLocals'],),
     'JSDCE': (['JSDCE', '--export-es6'],),
+    'sourcePhaseImports': (['JSDCE', '--export-es6'],),
     'JSDCE-hasOwnProperty': (['JSDCE'],),
     'JSDCE-defaultArg': (['JSDCE'],),
     'JSDCE-fors': (['JSDCE'],),

test/test_other.py

@@ -7,7 +7,7 @@ version published in npm.
 This `terser.js` bundle in this directory was built from our fork of terser
 which lives at: https://github.com/emscripten-core/terser/
 
-The current patches are stored in the `emscripten_patches_v4.8.0` branch.
+The current patches are stored in the `emscripten_patches_v5.18.2` branch.
 
 To make changes to this code please submit patches to
 https://github.com/emscripten-core/terser/ and then re-create this bundle